Some information from: www. floyd. k12. va Some information from: www.floyd.k12.va.us/itrt/pwrpnts/geography/Earthquakes.ppt www.iris.edu Earthquakes!

Seismicity & Distribution of Earthquakes There are thousands of small earthquakes every day “Strong” earthquakes (~M7) occur once a month. >M8 occur about once/year. Where are the deepest earthquakes? This figure shows the “baseball-stitching” pattern of earthquakes around the globe. This represents only a small percentage of the earthquakes that would typically occur in a year. Over a million earthquakes of Magnitude 2 and lower occur every year. And 1,500 Magnitude 5 occur every year. RESOURCE: For a one-page flier on “How often do earthquakes occur?” go to http://www.iris.edu/edu/onepagers.htm With some imagination you can see some of the gross continental boundaries. Worldwide, strong earthquakes happen more than once per month. Smaller earthquakes, such as magnitude 2 earthquakes, occur several hundred times a day. The deepest earthquakes are where one plate is subducting beneath another plate. INTERACTIVE FLASH rollovers—This and the next slides are available from : DynamicPlanet-Earthquakes & Volcanoes http://www.iris.edu/hq/programs/education_and_outreach/animations/interactive ACTIVITIES: World Tectonic Map activity link on page: http://www.iris.edu/hq/programs/education_and_outreach/animations/11 Exploring Rates of Earthquake Occurance: http://www.iris.edu/hq/resource/explore_rates_of_earthquake_occurance For earthquakes of the past 2 weeks, go to http://www.iris.edu/seismon/

World Seismicity & Plate Tectonics Notice that the earthquakes coincide with plate boundaries, and the deepest quakes (blue) are in subduction zones. Question: Where would you expect to see volcanoes? This slide shows the relationship between plate tectonics and earthquake location. There are thousands more earthquakes every year than shown here. Many are in the middle of plates. Why would there be earthquakes in the middle of plates? Remember that the plates are sliding over a round globe and they crack, crumple, and creak as they adjust to the new position. All these adjustments cause earthquakes. For earthquakes of last 2 weeks, go to http://www.iris.edu/seismon/ INTERACTIVE FLASH rollovers : ﾊﾊﾊﾊDynamicPlanet-Earthquakes & Volcanoes http://www.iris.edu/hq/programs/education_and_outreach/animations Create your own maps at http://www.iris.edu/quakes/maps.htm Modified from USGS Graphics

What are Earthquakes? The shaking or trembling caused by the sudden release of energy Usually associated with faulting or breaking of rocks Continuing adjustment of position results in aftershocks

What is the Elastic Rebound Theory? Explains how energy is stored in rocks Everything has an elastic limit Rocks bend until the strength of the rock is exceeded Rupture occurs and the rocks quickly rebound to an undeformed shape Video! Area where break and movement occurs is fault Energy is released in waves that radiate outward from the fault

What causes faults? Stress between two plates Volcanism Type of stress changes type of fault Volcanism Cavern collapse/ subsidence Contemporary Physical Geology, H.L. Levin, Saunders Publ.

Types of Fault Normal Reverse Strike-slip

Types of fault: Normal Hanging wall moves down Due to tension Wasatch

Types of fault: Reverse Hanging wall moves up Due to compression Reverse_fault_480.mov - YouTube

Types of fault: Strike-slip Rocks move past one another Due to shear Strike-slip-fault_480.mov - YouTube

A lecture from IRIS 4.Fault Models Lecture - YouTube

The Focus and Epicenter of an Earthquake The point within Earth where faulting begins is the focus, or hypocenter The point directly above the focus on the surface is the epicenter

Seismographs record earthquake events

What are Seismic Waves? Response of material to the arrival of energy released by rupture Two types: Body waves P and S Surface waves R and L

Body Waves: P and S waves P or primary waves fastest waves travel through solids, liquids, or gases compressional wave, material movement is in the same direction as wave movement S or secondary waves slower than P waves travel through solids only shear waves - move material perpendicular to wave movement

Surface Waves: R and L waves Travel just below or along the ground’s surface Slower than body waves; rolling and side-to-side movement Very damaging to buildings

How is an Earthquake’s Epicenter Located? P waves arrive first, then S waves, then L and R Average speeds for all these waves is known After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

Where Do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the circum-Pacific belt (The Ring of Fire) most of these result from convergent margin activity ~15% occur in the Mediterranean-Asiatic belt remaining 5% occur in the interiors of plates and on spreading ridge centers more than 150,000 quakes strong enough to be felt are recorded each year

At convergent boundaries, focal depth increases along a dipping seismic zone called a Benioff zone

How is an Earthquake’s Epicenter Located? Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

How is an Earthquake’s Epicenter Located? Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter

How are the Size and Strength of an Earthquake Measured? Intensity subjective measure of the kind of damage done and people’s reactions to it isoseismal lines identify areas of equal intensity Modified Mercalli Intensity Map 1994 Northridge, CA earthquake, magnitude 6.7

How are the Size and Strength of an Earthquake Measured? Magnitude Richter scale measures total amount of energy released by an earthquake; independent of intensity Amplitude of the largest wave produced is corrected for distance and assigned a value on an open-ended logarithmic scale

Structure of Earth Behavior of waves tells us about the material Earth's Interior Interactives . Dynamic Earth . Intro Structure of the Earth

Delaughter et al, found that at the undergraduate level, only 5% of students responded that scientists knew about the Earth’s interior based on information from earthquakes.

Basic Structure of Earth

Basic Structure of Earth Speed and paths of seismic waves change as they travel through materials with different densities Earth is made up of layers of different materials P and S waves do specific things in the mantle, outer core and inner core

The Economics and Societal Impacts of Earthquakes Damage in Oakland, CA, 1989 Building collapse Fire Tsunami Ground failure

What are the Destructive Effects of Earthquakes? Ground Shaking amplitude, duration, and damage increases in poorly consolidated rocks

Can Earthquakes be Predicted? Earthquake Precursors changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the ground seismic dilatancy model seismic gaps

Can Earthquakes be Predicted? Earthquake Prediction Programs include laboratory and field studies of rocks before, during, and after earthquakes monitor activity along major faults produce risk assessments

Can Earthquakes be Controlled? Graph showing the relationship between the amount of waste injected into wells per month and the average number of Denver earthquakes per month Some have suggested that pumping fluids into seismic gaps will cause small earthquakes while preventing large ones